Recently, the predicted exhaustion of conventional energy sources such as oil and coal has brought about an increasing interest in alternative energy sources. In particular, a fuel cell, as an energy storage system, is advantageous in that it is highly efficient, does not discharge pollutants such as NOx and SOx, and the fuel used is abundant, and thus attracts much attention.
A fuel cell is a power generation system which converts chemical bond energy of a fuel and an oxidizing agent into electric energy. Typically, hydrogen, methanol or hydrocarbons such as butane are used as the fuel and oxygen is used as the oxidizing agent.
The most basic unit to generate electricity in the fuel cell is a membrane electrode assembly (MEA), which is composed of an electrolyte membrane, and an anode electrode and a cathode electrode formed on both surfaces of the electrolyte membrane. Referring to Reaction Scheme I illustrating a mechanism via which a fuel cell generates electricity (reaction scheme of the fuel cell in the case where hydrogen is used as the fuel), in the anode electrode, oxidation occurs to produce hydrogen ions and electrons and the hydrogen ions move through the electrolyte membrane to the cathode electrode. In the cathode electrode, oxygen (oxidizing agent), the hydrogen ions transferred through the electrolyte membrane react with electrons to produce water. Based on these reactions, electron transfer occurs in an external circuit.At anode electrode: H2→2H++2e−At cathode electrode: ½O2+2H++2e−→H2OOverall reaction: H2+½O2→H2O  [Reaction Scheme I]
Among fuel cells, a proton exchange membrane fuel cell (also referred to as a “polymer electrode membrane fuel cell”, PEMFC) provides high energy efficiency, high current density and power density, short driving period and rapid response to load variation. The proton exchange membrane fuel cell utilizes a proton exchange membrane and requires high proton conductivity, chemical stability, thermal stability at an operating temperature, low gas permeability, and in particular, superior mechanical strength as a membrane. Although membranes satisfying these requirements have been developed, clean manufacturing technology for the production of price competitive membranes is needed to make commercialization possible. Fluorine-based membranes such as Nafion (manufactured by Du Pont), Aciplex (manufactured by Dow membrane or Asahi Chemical) have disadvantages of decreased proton conductivity and high production cost in a low-humidity and high-temperature process. Accordingly, a great deal of research associated with non-fluorine polymers in which a polar group is introduced into a heat-resistant polymer as a base skeleton to provide functionalities of polymer electrolytes is actively made. Of these, poly(arylene ether) polymers having aromatic derivatives and ether bonds exhibit good heat resistance and chemical resistance, superior mechanical strength, excellent durability and low production costs.
However, dimensional stability, in consideration of the fact that polymer electrolyte membranes of fuel cells generate large amount of water, is a very important factor to be contemplated. Commonly, an electrolyte membrane should have a high ion exchange capacity (IEC) in order to have a high ionic conductivity. However, since ion exchange capacity (IEC) of the electrolyte membrane is directly related to water uptake, as water uptake increases, ion exchange capacity increases. As a result, dimension stability is deteriorated and film thickness increases, thus disadvantageously deteriorating overall performance of cells.
However, non-fluorine polymer membranes which efficiently solve the problem of dimensional stability, while maintaining superior performance, have been not yet known.